As a separator for use in a nonaqueous electrolyte secondary battery such as a lithium ion secondary battery, there has been used (i) a microporous film which contains a polyolefin as its main component or (ii) a laminated porous film obtained by stacking a functional layer on the microporous film which serves as a base material.
In a process of producing such a film, the film is longitudinally or transversely stretched so as to control a pore structure of the film. Patent Literature 1 discloses that a polyolefin microporous film suitable for a base material porous film serving as a separator can be produced with high productivity, by causing a plurality of stretching regions in a stretching furnace of a film-stretching apparatus to have respective different film widening speeds and respective different temperatures such that the film widening speeds and the temperatures are in a specific relationship.